Finally, the map. I actually used quite an easy method to make the main map mesh. I got the height map of one part of the Grand Canyon from a free to use online tool, and then edited the map in GIMP so that the canyon drops off quickly at specific spots so that I could make waterfalls at those spots. Here's a top view of the map in the game along with the height map:

Original height map:

Map height map (edited for waterfall-dropoffs):

Current version of the map in Godot:

I used the Terrain3D Addon for Godot to import the height map, and then used the terrain editor to get rid of all the side branches since all we wanted was the main river. I then used the Terrain3D addon to instantiate rocks, several types of grass, and cacti around the map, as well as smooth out some of the rough spots, and make a grass bowl at the end of the game (can be seen in the top right corner). Here are some screenshots of the inside of the map: